A.T. Robertson Commentary Mark 4:24

"And he said unto them, Take heed what ye hear: with what measure ye mete it shall be measured unto you; and more shall be given unto you." — Mark 4:24 (ASV)

What ye hear (τ ακουετε). Luke 8:18 has it "how ye hear" (πως ακουετε). Both are important. Some things should not be heard at all for they besmirch the mind and heart. What is worth hearing should be heard rightly and heeded.

With what measure (εν ω μετρω). See already in the Sermon on the Mount (Matthew 7:2; Luke 6:38).